High brightness light sources are needed for many applications including optical fiber illumination and image projection. In optical fiber illumination, particularly for telecommunications applications, light-emitting diodes (LEDs), and semiconductor diode lasers are the dominant light sources as described in the article of Hecht, which is attached hereto and incorporated herein by reference (see Hecht, Jeff, Back to Basics: Fiber-optic Light Sources, Laser Focus World, January 2000). The output power density of LEDs is generally too low for most fiber illumination applications. Semiconductor diode lasers have many favorable characteristics for fiber illumination. Inexpensive diode lasers are readily available in red or near-infrared wavelengths. However, semiconductor diode lasers suitable for many other applications are either not available or very expensive to produce.
For a variety of reasons, lasers and LEDs are rarely used as light sources for image projection The primary reason is the high cost of lasers and LEDs capable of producing the high total outputs needed, especially one to several watts of blue light. In addition, coherent light sources such as lasers can produce artifacts in many projection applications. For these reasons the dominant light source for projection is the arc lamp.
Arc lamps are capable of the brightness and total luminous output required for almost any projection need. Indeed arc lamps are partially responsible for the great success of the movie industry in the 20th century. However, arc lamps are considered too expensive for use in many consumer devices. In addition, the wide spread use of arc lamps in consumer devices would pose a new set of safety problems.
In an issued U.S. Pat. No. 5,469,018, which is incorporated herein by reference, a Resonant Microcavity Display was disclosed. A resonant microcavity display is a light source incorporating a thin film phosphor embedded in a microcavity resonator. The microcavity resonator consists of an active region surrounded by reflectors. The dimensions are chosen such that a resonant standing wave or traveling wave is produced by the reflectors. The methods described lead to the emission of strong and controlled radiative modes. This is in contrast to a bare thin film phosphor (which is not provided in a microcavity) which generates strong emission into waveguide modes (i.e., the emissions travel along the material), but only weak and diffuse radiative emissions (i.e., for example perpendicular to the material).
A light source is formed by coupling an excitation source to the microcavity structure. The phosphor inside the microcavity may be excited through several means including bombardment by externally generated electrons (cathodoluminescence), excitation by electrodes placed across the active layer to create an electric field (electroluminescence) or excitation using photons (photoluminescence).
Phosphors in general are restricted in the power density of excitation and emission due to multiple causes. Phosphors are typically insulating materials with relatively low thermal conductivities. In addition, many phosphors exhibit relatively long emission times which limit the number of photons each luminescence center may produce in a given time. Due to these restrictions, phosphors may rarely be excited at levels greater than 1 W per square cm resulting in a emission level rarely greater than 100 MW per square cm. For these reasons phosphor based devices have been difficult to utilize in high brightness applications such as fiber illumination or film projection.